1. Field of the Invention
The present invention relates to an optical head used for at least either recording or reproducing information by light to/from a recording medium, an optical information recording and reproducing apparatus configured by using such an optical head, and a focus error detecting method for detecting a focus error of light condensed onto a recording medium.
2. Description of the Related Art
In recent years, various optical information recording and reproducing apparatuses for at least either optically recording or optically reproducing information by using an optical information recording medium such as an optical disk have been being commercially available. Among them, an optical disk drive using an optical disk as a recording medium has been remarkably spread and its recording density is becoming higher and higher. As a playback-only optical disk drive, for example, a DVD drive capable of reproducing a DVD of which recording capacity has been increased to 4.7 GB that is about 7 times as large as that of a CD-ROM (Compact Disk-Read Only Memory) is already commercially available. The DVD has the same size (diameter of 120 mm) as a CD-ROM having a recording capacity of about 650 MB.
Generally, in an optical disk, a recording surface is formed on a transparent substrate and light for recording or reproducing emitted to an optical disk via an objective lens passes through the transparent substrate and is condensed on the recording surface. For example, a DVD drive adopts a differential phase difference method (DPD method) using a photodetector which is divided into four parts by a cross lattice. On the other hand, in order to minimize a beam spot on the recording surface, focusing error detection is performed. As the detection method, an astigmatism method, in which the same photodetector as that used for the tracking error detection is usable, is often employed.
In a CD drive, the NA (Numerical Aperture) of an objective lens is set to 0.45. In a DVD drive, in order to make the recording density higher, the NA of the objective lens is increased to 0.60. When the NA is increased, generally, aberration (mainly, coma aberration) in the case where a disk is tilted increases and a reproduction signal is degraded. Since the aberration is usually almost proportional to the cube of the NA and the thickness of the substrate, in the DVD, in order to suppress the aberration, the thickness of a disk is set to 0.6 mm which is the half of that of a CD (=1.2 mm).
The DVD is expected as an optical disk which succeeds the CD. It is desired that the DVD drive is usable for reproducing information from a CD. In the case of reproducing a CD by an optical system including an objective lens optimized for reproducing a DVD of a higher density, spherical aberration occurs due to different thickness of the substrate. A good reproduction signal cannot be therefore obtained.
Under present circumstances, the DVD drive is further expected to reproduce also a CD-R (CD recordable) as a write-once CD. The CD-R is, however, usually made of materials including a coloring agent sensitive to light of a relatively long wavelength. Consequently, it is difficult to reproduce a CD-R by using a light source which emits light of 650 nm used in the DVD drive. In each of a DVD-ROM reproducing apparatus and a DVD-Video reproducing apparatus which can reproduce a CD-R as well, two kinds of light sources of a light source for emitting light of 650 nm and a light source for emitting light of 780 nm are used and an optical head (optical pickup) devised to suppress also signal degradation caused by different substrate thickness is adopted. An optical head for use in the DVD-ROM apparatus capable of reproducing not only a DVD-ROM but also a CD and a CD-R will be described hereinbelow. In the following description, a CD and a CD-R will be simply described as a CD, and a DVD-ROM and a DVD-Video will be simply described as a DVD.
FIG. 1 is a plan view showing the structure of a photodetector used for an optical head of a DVD apparatus capable of reproducing both a DVD and a CD.
First, the photodetector will be described. As shown in FIG. 1, a photodetector 19 comprises: a substrate 191; a photoreceiving part 192 for a main spot disposed in the central region of the substrate 191; and two photoreceiving parts 193 and 194 for side spots disposed at equal intervals on both sides of the photoreceiving part 192 for the main spot. The photoreceiving part 192 for the main spot has a rectangular shape as a whole and is divided in four photoreceiving regions 192A, 192B, 192C and 192D of almost the same shape by dividing lines of a cross. Each of the photoreceiving parts 193 and 194 for side spots is a single region which is not divided.
At the time of reproducing a CD, a light beam emitted from the light source for a CD (not shown) is split into three beams by a diffracting optical system (not shown). The three beams are condensed by an objective lens onto the recording surface of the CD as a recording medium. As shown in FIG. 1, the three light beams reflected by the recording surface of the CD enter the center areas of the photoreceiving part 192 for the main spot and the photoreceiving parts 193 and 194 for side spots in the photodetector 19 and form beam spots 196, 197 and 198, respectively.
On the other hand, at the time of reproducing a DVD, a light beam emitted from a light source for DVD (not shown) is condensed by the objective lens onto the recording surface of a DVD as a recording medium. The light beam reflected by the recording surface of the DVD passes through a predetermined optical system, is incident on the central area of the photoreceiving part 192 for the main spot in the photodetector 19, and forms the beam spot 196. The center of the beam spot 196 is adjusted to almost coincide with the center of the photoreceiving part 192 for the main spot (that is, the intersecting point of the four photoreceiving regions 192A, 192B, 192C and 192D). FIG. 1 shows the case where the beam spot 196 on the photoreceiving part 192 for the main spot has an almost circle shape, that is, the optical head is in the focusing state. The focusing state denotes a state where a light beam is focused by the objective lens to form the minimum spot on the recording surface of the recording medium. When the optical head goes out of focus, the beam spot 196 of the photoreceiving part 192 for the main spot changes its shape to either an oval shape having the major axis whose upper part is inclined to the left from the vertical line by 45 degrees and whose lower part is inclined to the right from the vertical line by 45 degrees as shown in FIG. 2A or an oval shape having the major axis whose upper part is inclined to the right from the vertical line by 45 degrees and whose lower part is inclined to the left from the vertical line by 45 degrees as shown in FIG. 2B. Each of FIGS. 2A and 2B enlargedly shows only the photoreceiving part 192 for the main spot in the photodetector 19.
When photoreception signals which go out from the four photoreceiving regions 192A, 192B, 192C and 192D of the photoreceiving part 192 for the main spot are designated by reference characters a, b, c and d, respectively, and photoreception signals which go out from the photoreceiving parts 193 and 194 for side spots are designated by reference characters e and f, respectively, a focus pull-in signal FPI, a reproduction signal RF, a focus error signal FE, and a tracking error signal TE are expressed by the following equations (1) to (4). The focus pull-in signal FPI is a signal used to regulate a range in which a focusing control is performed on the basis of the focus error signal FE and is obtained by, for example, eliminating high frequency components of the reproduction signal RF by using a predetermined low pass filter (not shown).
focus pull-in signal FPI=reproduction signal RF=a+b+c+dxe2x80x83xe2x80x83(1)
focus error signal FE=(a+c)xe2x88x92(b+d)xe2x80x83xe2x80x83(2)
tracking error signal TE=phase difference between (a+c) and (b+d)xe2x80x83xe2x80x83(3)
or
tracking error signal TE=exe2x88x92fxe2x80x83xe2x80x83(4)
The focus error signal FE expressed by the equation (2) is used for detecting a focus error by the astigmatism method. As described above, at the time of reproducing information from a DVD, in the photodetector 19 shown in FIG. 1, the shape of the beam spot 196 on the photoreceiving part 192 for the main spot becomes a circle or various ovals whose major axes are oriented differently in accordance with the degree of focusing. The focus error signal FE obtained by the equation (2) varies accordingly. More specifically, in the focusing state, output signals from the photoreceiving regions 192A, 192B, 192C and 192D of the photoreceiving part 192 for the main spot are almost equal to each other. Consequently, the focus error signal FE is almost zero. When the system is out of focus, the beam spot 196 has an oval shape. A difference therefore occurs between the sum (a+c) of the output signals from the photoreceiving regions 192A and 192C in one of diagonal line directions in the photoreceiving part 192 for the main spot and the sum (b+d) of output signals from the photoreceiving regions 192B and 192D in the other diagonal line direction. In this case, the sign of the difference between them depends on the direction of defocusing and the absolute value of the difference depends on the amount of defocusing. By moving the objective lens so that the focus error signal FE becomes zero, the best focusing state is maintained.
The method of tracking servo in the case of reproducing a DVD and that in the case of reproducing a CD are different from each other. Specifically, in the case of reproducing a DVD, a phase difference between (a+c) and (b+d) expressed by the equation (3) is obtained by using the signals which go out from the photoreceiving regions 192A, 192B, 192C and 192D of the central photoreceiving part 192 for the main spot used in the astigmatism method for focus servo. The phase difference is used as a tracking error signal TE. On the other hand, in the case of reproducing a CD, tracking servo is performed by the so-called three-spot method. According to the three-spot method, light to be applied on a disk is split into three light beams of the 0 order ray, +primary ray, and xe2x88x92primary ray by a diffraction grating or the like. The three light beams are received by the three receiving parts of the receiving part 192 for the main spot and the receiving parts 193 and 194 for the side spots. As a tracking error signal TE, (exe2x88x92f) expressed by the equation (4) is used.
The objective lens used for such an optical head has, for example, the structure as shown in FIG. 6. FIG. 6 shows, for convenience of explanation, a cross section of a CD in the right half and a cross section of a DVD in the left half. As shown in the diagram, in a region having the NA of about 0.38 to 0.44, an objective lens 27 has a zonal substrate thickness correcting part 27A which is recessed from the surrounding area. A central region 27B on the inside of the substrate thickness correcting part 27A and a peripheral region 27C on the outside of the substrate thickness correcting part 27A are formed in a surface shape optimum to the thickness (0.6 mm) of the substrate of a DVD-ROM. On the other hand, the substrate thickness correcting part 27A is designed so as to be in a surface shape optimum to the thickness (1.2 mm) of the substrate of a CD, thereby enabling aberration which occurs at the time of reproducing the CD to be corrected. The objective lens 27 of such a structure is driven in the direction which is orthogonal to the disk face by an actuator having a coil for driving the lens (not shown).
The wavefront aberration in the case of reproducing a CD by using the objective lens 27 of FIG. 6 is, for example, as shown in FIG. 7. In the diagram, the lateral axis indicates the NA of the objective lens and the vertical axis represents the spherical aberration (unit: mm). As shown in the diagram, the wavefront aberration in the case of reproducing a CD with the objective lens 27 is reduced to a degree that there is no problem in practical use by synthesis of a paraxial region where the spherical aberration is relatively small and a zonal region (substrate thickness correcting part 27A) designed for a CD. Since the spherical aberration is large in the peripheral part having the NA of 0.44 or larger, light is diffused and is not incident on the photoreceiving part 192 for the main spot (FIG. 1) of a small size which is usually used for detecting a focus error by the astigmatism method.
On the other hand, in the event of reproducing a DVD, light which passes the zonal region (substrate thickness correcting part 27A) is diffused and does not converge to the photoreceiving part 192 for the main spot. Consequently, an optical system having high compatibility capable of reproducing both a CD and a DVD by using a single objective lens and a single detecting device can be realized without hardly deteriorating the performance of reproducing a DVD. As a result, a simple-structured low-cost optical head for DVD, CD and CD-R, which has a small number of parts can be realized.
Recently, however, in addition to the above-described recording media, a rewritable DVD-RAM (Random Access Memory) has been put to a commercial use. A reproduction head for DVD which can reproduce also the DVD-RAM and a recording/reproducing optical head for DVD-RAM which can also reproduce the DVD and CD have come to be demanded.
A conventional reproduction-only DVD or CD employs the system of recording information on either a land or a groove. A DVD-RAM capable of recording and reproducing information employs a land/groove recording system for recording information on both lands and grooves in order to increase the recording density. Different from a conventional DVD or CD in which either the land or groove used for recording is widened and the other is narrowed, in the land/groove recording system, both of the land and the groove are designed to be wide to a certain degree.
In the land/groove recording system, however, in the case of detecting a focus error by using the astigmatism method, a phenomenon called xe2x80x9ctracking interferencexe2x80x9d which will be described hereinlater occurs. It has been confirmed that a noise called xe2x80x9ctrack crossing noisexe2x80x9d is caused by the phenomenon. The xe2x80x9ctracking interferencexe2x80x9d is a phenomenon such that a large change occurs in the focus error signal when a beam spot crosses a track. The xe2x80x9ctrack crossing noisexe2x80x9d is a noise caused by variations in the value of the focus error signal which occur depending on whether the beam spot is on a land or a groove in a recording medium.
FIG. 3 shows the xe2x80x9ctracking interferencexe2x80x9d phenomenon. In the diagram, the lateral axis shows the objective lens position in the direction which is orthogonal to a disk and the vertical axis indicates the output level of the focus error signal. A solid-line curve FEL is a focus error curve showing the relation between the objective lens position and the focus error signal FE in the case where the beam spot is on a land. A broken-line curve FEG is a focus error curve showing the relation between the objective lens position and the focus error signal FE in the case where the beam spot is on a groove.
As shown in the diagram, a range between the peaks of the focus error curve FEL (FEG) is specified as a focus pull-in range SPP. The focus servo is performed only in the range. The reason why the focus pull-in range SPP is provided and the focus servo is performed only in the range is that the focus error signal FE may become zero also in the case where the position of the objective lens is largely deviated from the focus position and it is necessary to eliminate the case where such a defocusing state is detected as a focusing state.
As shown in FIG. 3, depending on whether the beam spot is on the land or groove in the recording medium, the value of the focus error signal FE in the focus pull-in range SPP varies. Consequently, there are two positions at which the focus error signal FE becomes zero; an objective lens position XL in the case where the beam spot is on a land, and an objective lens position XG in the case where the beam spot is on a groove. On the other hand, a controller (not shown) for controlling the operation of the optical head controls a current passed to the coil for driving the lens (not shown) to drive the objective lens in the direction orthogonal to the disk so that the focus error signal FE becomes zero. Each time the beam spot moves from a land to a groove and from a groove to a land, the objective lens moves between the position XL and the position XG, and it appears as the track crossing noise. The noise causes inconveniences such as defocusing, deterioration in transmission characteristics in the focus servo and tracking servo, and burning or breakage of the coil for driving a lens. The mechanism of causing the tracking interference phenomenon explained by referring to FIG. 3 has not been fully analyzed.
In order to avoid the inconveniences caused by the track crossing noise, it can be considered to perform focus error detection by using what is called a spot size method. According to the astigmatism method as described above, the divided photoreceiving part 192 for the main spot (FIG. 1) is used and output signals from the divided regions are diagonally added and subtracted on the basis of the equation (2), thereby obtaining signals corresponding to the shape of the photoreceiving spot. In contrast, according to the spot size method, the spot size is detected from the output signals from the photoreceiving part and the focusing control is performed according to the spot size.
In the spot size method, however, since the photoreceiving part for the main spot having a relatively large photoreceiving area is necessary, it is accompanied by the following inconveniences. Specifically, as described above, in the case of performing the focus error detection in accordance with the spot size method by using the objective lens 27 (FIG. 6) having the zonal substrate thickness correcting part 27A to realize the low-cost optical head by sharing the objective lens and the photoreceiving part, since the photoreceiving part for the main spot is too large, diffusion light which has passed the peripheral region 27C is received by the photoreceiving part for the main spot at the time of reproducing a CD. On the other hand, at the time of reproducing a DVD, diffusion light which has passed the zonal substrate thickness correcting part 27A is received by the photoreceiving part for the main spot. Consequently, at the time of reproducing the CD or DVD, the reproduction signal RF is degraded and a servo signal such as a focus error signal is also degraded.
The invention has been achieved in consideration of the above problems and its object is to provide an optical head, a photodetector, an optical recording and reproducing apparatus and a focus error detecting method which can be adapted to a plurality of kinds of recording media and also to a land/groove recording system in which both lands and grooves are used as information recording regions.
According to the invention, there is provided an optical head comprising: a light source for emitting a light beam; an objective lens for condensing the light beam emitted from the light source onto a recording surface of a recording medium having a predetermined track guiding structure; light splitting means for splitting the light beam emitted from the light source and a light beam reflected by the recording surface of the recording medium from each other; photodetecting means for receiving the light beam reflected by the recording medium and split by the light splitting means; and astigmatism causing means for causing astigmatism in the light beam reflected by the recording surface of the recording medium and coming to the photodetecting means via the light splitting means, wherein the photodetecting means includes: four peripheral photoreceiving parts which are almost symmetrically arranged with respect to a first axis parallel to an arrangement direction of a plurality of diffraction patterns generated by the track guiding structure of the recording medium and a second axis perpendicular to the arrangement direction; and an intermediate photoreceiving part disposed in an intermediate region sandwiched by two arrangement lines parallel to the second axis in the arrangement of the four peripheral photoreceiving parts.
According to the invention, there is provided a photodetector for detecting a focus error which occurs when a light beam emitted from a light source is condensed on a recording medium having a predetermined track guiding structure by an objective lens, comprising: four peripheral photoreceiving parts which are almost symmetrically arranged with respect to a first axis which is parallel to an arrangement direction of a plurality of diffraction patterns which are generated by the track guiding structure of the recording medium and a second axis which is perpendicular to the arrangement direction; an intermediate photoreceiving part which is disposed in an intermediate region sandwiched by two arrangement lines parallel to the second axis in the arrangement of the four peripheral photoreceiving parts; an input terminal for inputting a switching signal; and switching means for switching output signals from the four peripheral photoreceiving parts and the intermediate photoreceiving part in response to the switching signal supplied from the input terminal.
According to the invention, there is provided an optical information recording and reproducing apparatus comprising: recording medium driving means for driving a recording medium; an optical head for reading a signal from the recording medium driven by the recording medium driving means; optical head driving means for moving the optical head along the recording medium; signal processing means for generating a reproduction signal on the basis of the signal read by the optical head; and servo control means for controlling operations of the recording medium driving means, the optical head driving means and the optical head on the basis of the signal read by the optical head, wherein the optical head has: a light source for emitting a light beam; an objective lens for condensing the light beam emitted from the light source onto a recording surface of a recording medium having a predetermined track guiding structure; light splitting means for splitting the light beam emitted from the light source and a light beam reflected by the recording surface of the recording medium from each other; photodetecting means for receiving the light beam reflected by the recording medium and split by the light splitting means, which has four peripheral photoreceiving parts almost symmetrically arranged with respect to a first axis parallel to an arrangement direction of a plurality of diffraction patterns generated by the track guiding structure of the recording medium and a second axis perpendicular to the arrangement direction, and an intermediate photoreceiving part disposed in an intermediate region sandwiched by two arrangement lines parallel to the second axis in the arrangement of the four peripheral photoreceiving parts; and astigmatism causing means for causing astigmatism in the light beam reflected by the recording surface of the recording medium and coming to the photodetecting means via the light splitting means.
According to the invention, there is provided a focus error detecting method used for an optical head having: a light source for emitting a light beam; an objective lens for condensing the light beam emitted from the light source onto a recording surface of a recording medium having a predetermined track guiding structure; light splitting means for splitting the light beam emitted from the light source and a light beam reflected by the recording surface of the recording medium from each other; photodetecting means for receiving the light beam reflected by the recording medium and split by the light splitting means; and astigmatism causing means for causing astigmatism in the light beam reflected by the recording surface of the recording medium and coming to the photodetecting means via the light splitting means, wherein the photodetecting means is equipped with: four peripheral photoreceiving parts which are almost symmetrically arranged with respect to a first axis parallel to an arrangement direction of a plurality of diffraction patterns generated by the track guiding structure of the recording medium and a second axis perpendicular to the arrangement direction; and an intermediate photoreceiving part disposed in an intermediate region sandwiched by two arrangement lines parallel to the second axis in the arrangement of the four peripheral photoreceiving parts, and a focus error signal is obtained by adding a detection signal obtained on the basis of signals outputted from the four peripheral photoreceiving parts and constant times of a detection signal obtained on the basis of signals outputted from the intermediate photoreceiving part. In this case, a constant in constant times may be not only a positive number but also a negative number and further zero.
In the optical head or the optical information recording and reproducing apparatus of the invention, the light beam emitted from the light source is focused on the recording surface of the recording medium having a predetermined track guiding structure by the objective lens and reflected by the recording surface. The light beam reflected by the recording surface of the recording medium is split from the light beam emitted from the light source by the light splitting means and comes onto the photodetecting means. At that time, astigmatism is given by the astigmatism causing means to the light beam from the recording surface of the recording medium. The photodetecting means includes: the four peripheral photoreceiving parts which are almost symmetrically arranged with respect to a first axis parallel to an arrangement direction of a plurality of diffraction patterns and a second axis perpendicular to the arrangement direction; and an intermediate photoreceiving part disposed in an intermediate region sandwiched by two arrangement lines parallel to the second axis in the arrangement of the four peripheral photoreceiving parts. The four peripheral photoreceiving parts and the intermediate photoreceiving part are irradiated with a light beam spot.
In the photodetector of the invention, output signals from the four peripheral photoreceiving parts which are almost symmetrically disposed with respect to the first and second axes and an output signal from the intermediate photoreceiving part disposed in the intermediate region sandwiched by two arrangement lines parallel to the second axis in the arrangement of the four peripheral photoreceiving parts are switched in response to the switching signal supplied from the input terminal. On the basis of the output signals, a focus error which occurs when a light beam emitted from the light source is condensed onto a recording medium having the predetermined track guiding structure by the objective lens is detected.
In the focus error detecting method of the invention, first, the photodetecting means is equipped with: the four peripheral photoreceiving parts which are almost symmetrically arranged with respect to the first axis parallel to an arrangement direction of a plurality of diffraction patterns generated by the track guiding structure of the recording medium and the second axis perpendicular to the arrangement direction; and the intermediate photoreceiving part disposed in an intermediate region sandwiched by two arrangement lines parallel to the second axis in the arrangement of the four peripheral photoreceiving parts. Then, a focus error signal is obtained by adding a detection signal obtained on the basis of signals outputted from the four peripheral photoreceiving parts and constant times of a detection signal obtained on the basis of signals outputted from the intermediate photoreceiving part.